Metal fuel pipes have been used on automobiles. Metal fuel pipes as fuel lines are plated or coated with a multilayer resin coating to improve the corrosion resistance and chemical resistance of the metal fuel pipes.
Recently, resin fuel pipes have been used as well as metal fuel pipes as fuel lines. Resin fuel pipes have many advantages over metal fuel pipes. Resin fuel pipes are not rusted, can be easily processed, are lightweight and increase the degree of freedom of design. Usually, resin fuel pipes are made of thermoplastic resins. Resin fuel pipes made of a polyamide resin (PA resin) are used most prevalently.
Resin fuel pipes made of a thermoplastic resin are inferior to metal fuel pipes in heat resistance. Therefore, it is usual to avoid using resin fuel pipes in a hot environment around the engine and to use resin fuel pipes as fuel lines connected to the fuel tank. Metal fuel pipes are used in the engine compartment.
Recently, need for using resin fuel pipes in the engine compartment has progressively increased and heat-resistant resin fuel pipes having improved heat resistance have been developed. Some heat-resistant resin fuel pipes have a part provided with a protector made of EPDM capable of protecting the resin fuel pipe from heat and some other heat-resistant resin fuel pipes are coated with a thermoplastic elastomer (hereinafter, abbreviated to “TPE”), such as a fire-retardant thermoplastic resin.
Although the protector made of EPDM is indubitably highly heat-resistant because EPDM is a thermosetting resin, it is difficult to put the protector made of EPDM on the resin fuel pipe because EPDM is rubber and hence has difficulty in smoothly sliding on the resin fuel pipe. Therefore, to put the protector on the resin fuel pipe, the protector must be formed so that an air layer is formed between the protector and the resin fuel pipe and the surface of the resin fuel pipe needs to be lubricated with a lubricant, such as silicone oil. Thus the use of the protector requires troublesome processes.
The resin fuel pipe coated with the fire-retardant resin, namely, a TPE, can be easily formed by coextrusion molding because the resin fuel pipe and the coating are made of thermoplastic resins, respectively. However, the fire-retardant TPE has a limited heat-resistant ability and the resin fuel pipe coated with a TPE is not suitable for use in a hot environment, such as an environment in an engine compartment in which the resin fuel pipe is exposed directly to heat.